(1) Field of the Invention
The invention is related to a compound rotorcraft with a fuselage, at least one main rotor that is at least adapted for generating lift in operation, and a fixed wing arrangement that is laterally attached to the fuselage.
(2) Description of Related Art
Compound rotorcrafts and so-called convertiplanes are basically the most relevant concepts aiming to overcome horizontal flight deficiencies of conventional helicopters, i.e. helicopters with a main rotor and an auxiliary tail rotor that is adapted to counter torque, by introducing attributes of fixed-wing aircrafts to such conventional helicopters as compromise. However, a compromise between both aircraft types has always to be conveniently adapted to a planned mission profile of a given rotorcraft.
An exemplary convertiplane is e.g. described in the document U.S. Pat. No. 5,046,684. More specifically, the latter describes a tiltrotor aircraft with a fuselage and a fixed wing arrangement. On each side of the fuselage a first and a second wing are arranged. The first wing is fixed at substantially the bottom of the fuselage and substantially unperforated in hovering as well as forward flight. The second wing is fixed at substantially the top of the fuselage, or fixed to a structure extending above the fuselage, and is likewise substantially unperforated in hovering as well as forward flight. At least one of the first and second wings has dihedral so that the wings converge to join or nearly join at their tips. Furthermore, unducted rotor means are provided for generating aerodynamic lift sufficient for highly efficient hovering flight and for propelling the tiltrotor aircraft at speeds approaching roughly four hundred knots in forward cruising flight. The unducted rotor means are supported on the first and second wings, at or near the tips of the first and second wings. They can be pivoted for operation in different orientations in hovering and forward flight respectively.
In other words, according to the document U.S. Pat. No. 5,046,684, the tiltrotor aircraft features two fully tiltable rotors, one at each side of the fuselage, and which are respectively arranged at the tips of a joined wing of the fixed wing arrangement. In this fixed wing arrangement, the lower, i.e. first wing is straight and positively swept, and the upper, i.e. second wing is straight and exhibits a very pronounced negative sweep. The upper wing is anhedral and connects the tip of the lower wing of the fixed wing arrangement to the tip of the tiltrotor aircraft's fin.
In contrast to such a tiltrotor aircraft, winged compound rotorcraft configurations with separate propulsion units typically feature a monoplane design with one set of wing surfaces in cantilever design as shoulder-wing arrangement. A compound rotorcraft with lift compounding, thrust compounding or a combination of both basically aims to off-load a respective main rotor from its simultaneous lifting and propulsive duties to allow for higher forward speeds of the compound rotorcraft.
More specifically, lift compounding entails adding wings to a rotorcraft, hence enabling to increase an underlying load factor of the rotorcraft and to reach a higher maneuverability. This improves the efficiency of the rotorcraft at moderately high speed but at the expense of reduced efficiencies at lower forward speeds and in the hover.
Thrust compounding implies the addition of essentially horizontally oriented auxiliary propulsion units to the rotorcraft. This has been typically accomplished by means of a single or a pair of propellers being driven by drive shafts powered by main turboshaft engines of the rotorcraft. The use of a pair of propulsion units has the advantage of providing for anti-torque capabilities without the need of an additional tail rotor, hence relativizing the inherent system complexity of the thrust compound configuration.
A more extended configuration of a compound rotorcraft includes both the addition of wings and propulsion units. In this case, lift during cruise is simultaneously provided by a given main rotor—in powered condition—usually addressed as “hybrid helicopter”—or in autorotation—“autogyro”—modus—and the wings. Higher forward speed is provided by horizontally oriented auxiliary propulsion units of the compound rotorcraft. The compound rotorcraft hence overcomes underlying rotor lift limits by means of the wings and underlying rotor thrust limits by means of the propulsion units. As a result, a higher load factor is obtained along with potential for higher speed. In particular, use of a pair of thrust propulsion units—opposed and both offset relative to each other and to a longitudinal axis of the compound rotorcraft—enables for a simultaneous torque correction.
Exemplary compound rotorcrafts with two wing-mounted propellers defining the above-described propulsion units are described in the documents EP 2146896, EP 2690011 and US 2013/0175385. These exemplary compound rotorcrafts are all provided with fixed wing arrangements, as described hereinafter.
The document EP 2146896 describes a compound rotorcraft with a fixed wing arrangement in the form of a cantilevered wing configuration that comprises pusher propellers that are installed at tips of straight and plane cantilevered wings with associated drive shafts that are respectively housed within the wings. Corresponding housings of the propeller drive shafts that are arranged inside the wings require an underlying wing planform to be straight. Furthermore, the cantilevered wing configuration requires a large wing root thickness and a continuation of wing bending capabilities throughout an upper fuselage deck. In general, such a cantilevered wing configuration comprises straight wings, wherein each of the wings has a constant sweep and constant anhedral.
The document EP 2690011 describes a compound rotorcraft with a fixed wing arrangement in the form of a joined-wing configuration, wherein a lower wing and an upper wing are provided on each side of the compound rotorcraft. Both wings are essentially straight and interconnected to each other at a wing interconnection region, and a pusher propeller is installed in the interconnection region behind associated trailing edges of both wings. This joined-wing configuration especially outstands by its improved mechanical efficiency in terms of less structural weight and larger stiffness, as well as by improved inherent operational safety characteristics and improved system integration, especially referring to an underlying accessibility of a main gear box of the compound rotorcraft.
The document EP 2418148 describes an airliner aircraft having a lambda-box wing configuration. The aircraft comprises a fuselage, a propulsion system with turbojets, a first pair of swept-back airfoils, connected to the top forward portion of the fuselage. A second pair of swept-forward airfoils is connected to the lower rear portion of the fuselage at a point of the fuselage aft of the connection of the swept-back airfoils. A third pair of substantially vertical airfoils is provided between the tips of the swept-forward airfoils and the lower side of the swept-back airfoils.
The propulsion system has one engine mounted in the extension middle of each of the first swept-back airfoils, either directly or on top of a further stand. The swept-back airfoils extend with an outwardly upwards angle with respect to the horizontal plane. The second pair of swept-forward airfoils are frankly shaped.
The document U.S. Pat. No. 4,856,736 describes an aircraft having paired aerofoils where wing tips are joined in the same plane one behind the other. The trailing edge of the forward wings and the leading edge of the rearward wings is coincidental in plan view at the wing tip. A single propeller is mounted at a front end of the fuselage in an embodiment. A pair of propellers is mounted at a front end of the fuselage, each on a canard pole, in another embodiment.
The document U.S. Pat. No. 6,098,923 describes a jet aircraft structure that provides inherent directional stability, even at very high angles-of-attack where conventional means of stabilization are ineffective. Components attached to an aircraft fuselage include a wing, horizontal stabilizers and vertical stabilizers. On the fuselage, are provided a front inlet and a rear exhaust nozzle.
The document U.S. Pat. No. 2,290,850 describes a folding wing airplane, where end sections of each wings can be folded upwards. The fuselage includes a front central propeller.
The document U.S. Pat. No. 8,657,226 describes synergistic control enhancement and drag reduction benefits in an aircraft having independent airfoils producing downward force opposite to wing lift in normal flight. The airfoils are supported in specific wingtip locations. The aircraft is of fixed wing type, with propellers mounted on the fuselage, on poles in some embodiments.
The document US 2013/0175383 describes another compound rotorcraft with fixed wing arrangement in the form of a joined-wing configuration having lower and upper wings that are each parallel to a given pitch axis of the compound rotorcraft. The wings exhibit a constant dihedral and the upper wings entirely cover the lower wings so as to minimize down-wash drag. This translates to a design with same depth of the wings, same wing orientation, and same position of attachment of the upper and lower wings.
However, all of the above-described compound rotorcrafts have drawbacks when used as high-speed rotorcrafts, as their fixed wing arrangements are not optimized with respect to high-speed regime in operation of the compound rotorcrafts. Furthermore, the respective wings of the fixed wing arrangements are not adequate for main landing gear integration and they can be improved with respect to wing interconnection and wing-to-fuselage transition. Moreover, the operational efficiency of these compound rotorcrafts as a whole can be improved.